For the reaction $\mathrm{A}+\mathrm{B}\to \mathrm{C}$, it is found that doubling the concentration of A increases the rate by 4 times, and doubling the concentration of B doubles the reaction rate. What is the overall order of the reaction? 

$\mathrm{A}+\mathrm{B}\to \mathrm{C}$

On doubling the concentration of A rate of reaction increases by four times. Rate $\propto {\left[\mathrm{A}\right]}^2$

However on doubling the concentration of B, rate of reaction increases two times. Rate $\propto \left[\mathrm{B}\right]$

Thus, overall order of reaction = 2 + 1 = 3.